1. Technical Field
The present invention relates to a mobile communication system comprising a plurality of communication units known as mobile stations and base stations, and more particularly, relates to an output control unit of a mobile communication system and its method for controlling an output characteristic of a transmit up-converter board.
2. Related Art
Modem mobile communication systems disclosed, for example, in U.S. Pat. No. 5,321,847 for Apparatus And Method For Detecting Intermodulation Distortion In A Radio Frequency Receiver issued to Johnson, Jr., and U.S. Pat. No. 5,524,285 for Radio Transmitter With Power Amplifier And Linarization issued to Wray et al., in which a plurality of communication units known as mobile stations and base stations communicate with each other over a wide band of frequencies within a single communication channel, are commonly known to include an upconverter to up-convert a frequency of a transmission signal and a downconverter to down-convert a frequency of a reception signal for communication purposes. Separate local oscillators for an upconverter are used to up-convert the frequency of a transmission signal at different stages of the uplink for transmission. Various automatic gain control circuitry such as disclosed in U.S. Pat. No. 4,665,560 for Wide Band Automatic Gain Control With Limiting Amplifiers issued to Lange, U.S. Pat. No. 5,301,364 for Method And Apparatus For Digital Automatic Gain Control In A Receiver issued to Arens et al., U.S. Pat. No. 5,321,849 for System For Controlling Signal Level At Both Ends Of A Transmission Link Based On A Detected Valve issued to Lemson, U.S. Pat. No. 5,630,220 for Automatic Gain Control Circuit For Radio Communication Apparatus issued to Yano, and output power control circuitry such as disclosed in U.S. Pat. No. 5,193,223 for Power Control Circuitry For A TDMA Radio Frequency Transmitter issued to Walczak et al., U.S. Pat. No. 5,287,555 for Power Control Circuitry For A TDMA Radio Frequency Transmitter issued to Wilson et al., are used to reduce saturation and noise.
Generally, an input signal for an upconverter or a downconverter is converted in two different stages using different local oscillators. The first stage requires the input signal to convert into a first intermediate frequency signal in a low frequency band which is then gain controlled and amplified for subsequent final conversion at the second stage using a relatively high local oscillation frequency. Such conversion arrangement has, as I have noted, resulted in unwarranted generation of undesirable signals due to the relatively high local oscillation frequency used. Moreover, temperature sensitivity, manufacturing variations and poor linearity of circuit components still attribute substantial degradation in performance.